1. Field of the Invention
The present invention relates to an electrostatic occupant detecting apparatus for detecting a seating of an occupant of a vehicle. The present invention also relates to a method of adjusting an electrostatic occupant detecting apparatus.
2. Description of the Related Art
JP-A-11-271463 discloses an electrostatic occupant detecting apparatus that includes an electrostatic sensor and an occupant detecting electronic control unit (occupant detecting ECU). The electrostatic sensor has a mat shape. The electrostatic sensor detects a change in electric field that generates between a main electrode disposed in a seat and a vehicle body and outputs a detected result as electric current or voltage.
The electrostatic sensor includes a pair of electrodes. When the seat is empty, air is inserted between the pair of electrodes. When a child restraint system (CRS) is disposed on the seat, the CRS is inserted between the pair of electrodes. When an occupant sits on the seat, the occupant is inserted between the pair of electrodes.
Air has a relative permittivity of about 1. The CRS has a relative permittivity depending on materials of the CRS and the relative permittivity is from about 2 to about 5. A human body has a relative permittivity of about 50. Because air, a CRS and a human body have different relative permittivities, an electrostatic capacity between the pair of electrodes of the electrostatic sensor varies by an object inserted between the pair of electrodes.
The electric field between the pair of electrodes changes due to a difference in the electrostatic capacity. The electrostatic sensor detects the change in the electric field between the pair of electrodes and outputs a detected result as electric current or voltage. The occupant detecting ECU determines whether the seat is empty, whether the CRS is disposed on the seat, and whether an adult sits on the seat based on an electric current value or a voltage value from the electrostatic sensor. An airbag ECU controls an inflation/non-inflation of an airbag based on a determination result of the occupant detecting ECU. When the seat is empty or when a CRS is disposed on the seat, the airbag ECU prohibits an inflation of the airbag. When an adult sits on the seat, the airbag ECU allows an inflation of the airbag.
US 2005/0275202 A (corresponding to JP-A-2006-27591) discloses an electrostatic sensor that detects wetness of a seat so as to discriminate a case where an occupant sits on the seat from a case where the seat is empty with a high degree of accuracy.
Water has a relative permittivity of about 80. Because the relative permittivity of water is greater than the relative permittivity of a human body, when the seat is wet, the occupant ECU is difficult to determine whether an occupant sits on the seat. Thus, a sub electrode is provided in the electrostatic sensor for detecting wetness. The electrostatic sensor detects a change in electric field between the sub electrode and the main electrode disposed in the seat and outputs a detected result as electric current or voltage.
US 2006/0164254 A (corresponding to JP-A-2006-201129) discloses an electrostatic sensor that includes a guard electrode for reducing a capacity. When a seat is empty, the electrostatic sensor reduces electric current flowing between a pair of electrodes used for determining a seating of an occupant. When an occupant sits on the seat, the electrostatic sensor can detect electric current flowing between the pair of electrodes with certainty.
US 2006/0164254 A further discloses an electrostatic sensor that detects electric current flowing between the guard electrode and the main electrode and detects abnormality of a capacitor disposed between the guard electrode and the main electrode based on the detected electric current.
Each of the above-described electrostatic sensors detects a change in electric field generating between predetermined electrodes and outputs a detected result as electric current or voltage. That is, the electric field between the predetermined electrodes depends on whether an occupant sits on the seat, whether the seat is wet, and whether the capacitor has an abnormality, and each of the electrostatic sensors outputs electric current or voltage having a level in accordance with the electric field. Then, the occupant detecting ECU determines whether an occupant sits on the seat, whether the seat is wet, and whether the capacitor has an abnormality based on an electric current value or a voltage value from the electrostatic sensor.
The electric current value or the voltage value depends on the electrostatic capacity between the predetermined electrodes and a resistance component of a circuit in the electrostatic sensor. That is, when the electric current value or the voltage value between the predetermined electrodes is detected with the electrostatic sensor, the detected electric current value or the detected voltage value depends on the resistance component of the circuit. The resistance component includes a resistance of, for example, an occupant, water, and air inserted between the predetermined electrodes. This is because that each of a human body, water, and air corresponds to a parallel circuit of a resistor and a capacitor in an equivalent circuit.
When the electrostatic sensor detects the electric current flowing between the predetermined electrodes, and whether an occupant sits on the seat, whether the seat is wet, and whether the capacitor has an abnormality are determined based on the detected electric current value, the detected electric current value includes an electric current value flowing in the parallel circuit of the resistor and the capacitor between the predetermined electrodes. Thus, if the occupant ECU determines whether an occupant sits on the seat and whether the seat is wet based on the detected electric current value as it is, an accuracy of the determination is limited because the detected electric current value does not depend on only pure electrostatic capacity between the predetermined electrodes.
US 2008/0100425 A discloses an electrostatic occupant detecting sensor that includes a power source, a main electrode, a main wiring part, a guard electrode, an impedance calculating part, a real and imaginary term calculating part, and a determining part. The power source generates an alternating-current (AC) voltage. The main electrode is disposed at a seating part of a seat of a vehicle. The main wiring part couples the power source and the main electrode. The guard electrode is disposed between a seat frame coupled with a vehicle ground and the main electrode. The guard electrode is spaced from the main electrode and opposes the main electrode. The guard electrode restricts generation of an electric field between the seat frame and the main electrode.
The impedance detecting part calculates an impedance from the power source to the main wiring part, the main electrode, and the vehicle body. The real and imaginary term calculating part calculates a real term and an imaginary term of the calculated impedance. The determining part determines whether an occupant sits on the seat based on the imaginary term in the calculated impedance.
The power source applies an AC voltage to the main electrode through the main wiring part, and thereby electric field generates between the main electrode and the vehicle body. The impedance calculating part calculates an impedance of an occupant detecting circuit from the power source to the main wiring part, the main electrode, and the vehicle body. In addition, the real and imaginary term calculating part calculates a real term and an imaginary term of the calculated impedance. In the occupant detecting circuit, the imaginary term corresponds to an electrostatic capacity of the capacitor in the parallel circuit of the resistor and the capacitor provided by an object such as a human body located between the main electrode and the vehicle body. The determining part determines an occupant on the seat based on the imaginary part of the impedance.
In the above-described electrostatic occupant detecting sensor, the imaginary term of the impedance corresponding to the electrostatic capacity between predetermined electrodes is used as a determinating factor. Thereby, the electric capacity between the predetermined electrodes can be calculated more accurately.
However, a sensitivity and a zero-point of the electrostatic sensor in the electrostatic sensor may vary among vehicles. Thus, a determination accuracy of an occupant may vary among vehicles.